1. Field of the Invention
The present invention relates to a process and apparatus for the analysis of a printed material and, more particularly, to a process and apparatus for accurate colorimetric analysis of a printed color measuring strip to produce color information from the printed color measuring strip by scanning the strip with a densitometer in an accurately controlled manner.
2. State Of The Prior Art
Devices of the type set forth herein, socalled densitometers are widely used, particularly in the printing industry. They make it possible for the printer to objectively evaluate the quality of a printed product, and to correspondingly set or adjust the printing apparatus to obtain a desired result, by means of the colorimetric analysis of a co-printed color measuring strip and the comparison of the measured values with those desired. Details of such densitometer measuring systems may be found in the prior art literature and, in particular, in U.S. Pat. Nos. 4,200,932 and 3,995,958 and the references cited therein.
For obvious reasons, it is desirable to automate the measuring and evaluation of the densitometric color measuring strips as far as possible. Consequently, a number of different scanning densitometers have become available for this purpose. Typically, they are equipped with a measuring head and appropriate controls to move the head back and forth on a bridge extending over the printed sheet, and thus they are capable of scanning and taking measurements from the strip in a more or less automatic manner.
A fundamental problem in the automatic measurement of densitometric strips is the detection and identification of the individual measuring fields of the strip and the determination of a suitable measuring position in order to obtain correct correlation of the measuring signals from the strip with the individual measuring fields. In this regard, it would be most desirable if the measuring head and its electronic controller could together provide appropriate orientation from the information contained in the measuring strip alone, without resort to the unusual, and as a rule undesirable, special situation in which auxiliary means such as synchronizing symbols or the like are used.
In the case of the scanning densitometer described in No. DE-A-29 01 980, the problem is addressed by continuously scanning the measuring strip and twice differentiating the measuring signal produced. The two differentiations produce zero crossings of the signal at the boundaries of the measuring fields. A location following a zero crossing at a certain, predetermined distance from the crossing, and thus from the boundary of a measuring field, is defined as the center of the field and thus as a suitable measuring location. To correct simply the aperture errors occurring in this mode of operation, color filters preceding the measuring head are changed in relation to the structure of the densitometric strip to be measured, thus assuring that only jumps in density in one direction are considered in the detection of measuring field boundaries.
With this known densitometer, the detection of measuring field boundaries is assured only if solid fields of an adequately high density and constantly changing colors follow each other in the measuring strip. A further, much more important disadvantage lies in the fact that it is necessary for the densitometer to have information as to both the configuration and the structure of the measuring strip and the nominal lengths of the measuring fields, so that the color filters may be changed in an appropriate manner and the centers of the fields determined as the measuring positions.
In U.S. Pat. No. 3,995,958, another scanning densitometer operating automatically to some extent, is described. This densitometer scans the measuring strip in discrete steps, and the determination of a measured change in color density of a certain value during two successive scanning steps is utilized to indicate a transition between two "blocks" with different color properties on the measuring strip. The value measured at a location following the point of transition determined in accordance with the abovementioned criterion is chosen as the valid measured value of color density for a given block. Following each block transition, a color filter change is effected, in order to insure an adequate jump in density for the next block transition.
A disadvantage of this known densitometer resides in the requirements of a highly specific configuration of the color measuring strip, i.e., a cyclic sequence of overlapping solid or empty blocks. Furthermore, the color sequence of the individual blocks must be known and supplied to the densitometer or to its controller. The lengths of the individual blocks must also be known in order to determine effective measuring locations on the basis of "block" transition detected in this relatively inaccurate manner.